ON THE OPERATION OF X-RAY POLARIMETERS WITH A LARGE FIELD OF VIEW

Abstract

The measurement of the linear polarization is one of the hot topics of High Energy Astrophysics. Gas detectors based on photoelectric effect have paved the way for the design of sensitive instruments and mission proposals based on them have been presented in the last few years in the energy range from about 2 keV to a few tens of keV. As well, a number of polarimeters based on Compton scattering are approved or discussed for launch on-board balloons or space satellites at higher energies. These instruments are typically dedicated to pointed observations with narrow field of view telescopes or collimators, but there are also projects aimed at the polarimetry of bright transient sources, like Soft Gamma Repeaters or the prompt emission of Gamma Ray Bursts. Given the erratic appearance of such events in the sky, these polarimeters have a large field of view to catch a reasonable number of them and, as a result, photons may impinge on the detector off-axis. This changes dramatically the response of the instrument to polarization, regardless if photoabsorption or Compton scattering is involved. Instead of the simple cosine square dependency expected for polarized photons which are incident on-axis, the response is never purely cosinusoidal and a systematic modulation appears also for unpolarized radiation. We investigate the origin of these differences and present an analytic treatment which proves that actually such systematic effects are the natural consequence of how current instruments operate. Our analysis provides the expected response of photoelectric or Compton polarimeters to photons impinging with any inclination and state of polarization.